Keep in mind that for the "Exceptionally increased strength, speed, reflexes, and endurance" would simply be from a commoner viewpoint of what all characters in ACKs would be like when they get a number of levels under their belts (especially those with fighter/ Hit Dice ranks).
Witchers in general I'd say wouldn't really be out and about much alone without a bunch of levels up their sleves (they seem to have quite the intense training). The earilest going out on their own at around 3rd to 5th level. While someone like Geralt would be "name" level (so around 9th level or more).
A class something akin to Fighter 2, HP 1 and Wizard/Thief 1 (to cover the magic skills) could cover most of what the witchers could do.
That said a custom race could work as well.